1. Field of the Invention
This invention relates to universally applicable backer rods for expansion joints, and, more particularly, for masonry expansion joints, a backer rod that controls the depth of sealants used therein.
2. Background Information
In the past standard backer rods for sealant control have been developed from closed cell polyethylene foam. These standard backer rods are extruded round rods of low-density, polyethylene (LDPE) material with a self-skinned outer covering. Upon use and prior to a sealant being applied, masonry backer rods are inserted into joints between two cured masonry sidewalls. In nominal terms, the backer rod, which has a diameter approximately 125% of the joint width, is compressively inserted into the joint and is installed at a depth of one-half the width thereof.
The standard backer rod is inserted in the expansion joint as hereinbefore described and the cavity remaining above the backer rod is filled with a cold sealant. In practice, the cavity is generally slightly overfilled with the sealant and the excess removed in an additional installation step. Over time with the impact of usual ambient conditions, air bubbles are released from the cells of the standard backer rod foam and migrate to the surface of the sealant.
The air bubbles released from the foam are described in the technical literature as outgassing. This is somewhat misdescriptive as the blowing gases used to create the foam have dissipated during curing. The appearance of bubbles at the surface of an expansion joint spoils the surface integrity and is unappealing. Also, such appearance indicates a possible failure of the seal provided by the underlying backer rod as the migrating air bubbles leave behind collapsed foam cells The collapsed cells of the backer rod change the compression and recovery characteristics thereof which need to be maintained during the expansion and contraction of the construction elements (or sidewalls).
To meet architectural standards and building code requirements, the installed standard backer rod needs to exhibit several specific physical properties. The first of these is inertness so that the self-skinned, polyethylene foam structure does not react with to the sealant materials. In the past it was also believed that the sealant and the backer rod should operate independently from one another and thus, besides being inert, it was desired that the backer rod not adhere to the sealant. The backer rod also must be stable over a broad temperature range, provide low water absorption, and adequate insulative qualities. When the seal of the backer rod materials in the expansion joint fails, the failure often is indicated by the previously described outgassing which over time produces unsightly bubbling at the surface of the sealant.
As an alternative to the standard backer rod, a temperature-resistant backer rod formed from crosslinked, closed-cell polyethylene has been marketed for use with hot-pour sealants. This backer rod is rated to temperatures in the range of 400° F. and is chemically inert resisting oil, gasoline and other solvents. The outer skin being of crosslinked polyethylene is smooth and impenetrable by the hot-pour sealant.
A further alternative to the standard backer rod is a soft rod or cool rod specifically designed as a compressible backing material for use in conjunction with cold-applied joint sealants. In this configuration the backer rod is a mix of closed- and open-cell structures and is substantially uniform throughout. Within the restrictive use, the soft rod design is promoted as being a non-outgassing structure.
Under the jurisdiction of ASTM Committee C-24 on Building Seals and Sealants, a standard has been developed for the construction industry entitled Standard Test for Determining the Outgassing Potential of Sealant Backer, ASTM Designation C 1253-93 (Reapproved 1998). According to ASTM, this test method provides an indicator of a potential sealing problem that could occur if a sealant backer is flawed in manufacture or transportation to the job site or is abused during the installation.
The testing reveals that some below-standard sealant backer materials, upon exposure to certain environmental conditions, can outgas, and create voids in the applied sealant before the sealant cures. The ASTM Standard further suggests that such voids potentially compromise the intended performance of the cured sealant. Voids are also known to be caused by other means and under certain conditions, such as air entrapment during sealant application, trapped air in the substrate releasing into the uncured sealant, incompatibility of the sealant with the sealant backer or substrate, or inhospitable installation conditions in the field, Therefore, the C 1253-93 test method is limited to identifying the outgassing potential of a punctured sealant backer by formation of a void in the soft uncured sealant under conditions of heat and compression.
In addition to the standard backer rod material of closed cell polyethylene, supra, and in preparing for this application, the inventor hereof came to know U.S. Pat. No. 5,277,515 of Hovis, et al., dated Jan. 11, 1994, and entitled, Extruded ethylenic polymer foam containing both open and closed cells. The Hovis '515 patent, one of several related Hovis patents, discloses an ethylenic polymer foam for a sealed building joint. The material is an extruded, low density, non-outgassing ethylenic polymer foam containing 30–85% open cells and the remainder closed cells with a self-skinned exterior surface. In the patent, the compression recovery and lower compression resistance of the foam is presented as being superior to similarly comprised ethylenic closed cell foams. Additionally, the foam is presented as having superior moisture resistance and handling characteristics when compared to open cell foams.
The prior art also provides several examples of roadway expansion joint structures and methods of use. Gibbon et al., U.S. Pat. No. 4,699,540 describes an expansion joint having a tube-shaped preform, preferably of a heat-cured silicone rubber, for insertion in the expansion joint as a sealing element. Gibbon '540 takes a silicone elastomer, which is pigmented and then catalyzed with a peroxide catalyst. The composition is extruded through a die followed by heat treatment either by heating in an air tunnel or heating in a salt bath.
Another backer rod used in roadway expansion joints is that of Dietlin et al., U.S. Pat. No. 5,007,765. Here, similar to the standard backer rod, supra, the inventor describes an open cell foam with an impervious skin. Because of its resiliency, the open cell foam can be sealed against any deformities or projections on the joint surface. In roadway expansion joint applications in which flowable sealants are used, the resiliency of the backer rod provides a seal against rough surfaces. However, as described below the backer rod of this invention provides a material better suited to masonry applications and substantially vertical expansion joints.
The term outgassing is a misnomer implying that the backer rod contains some sort of mysterious gas when in fact the closed cell backer rod cells contain only atmospheric air at atmospheric pressure. If a closed cell backer rod is torn or punctured during installation and sealant is applied, a bubbling situation in concert with a number of other factors may arise. This can occur as the sun warms up the joint causing the structure being sealed to expand and compress the rod squeezing the entrained air in the tear or puncture into the sealant. This warming can also cause the air in the void itself to expand due to thermal expansion and if this heating continues and a low viscosity slow curing sealant is used bubbling may occur. This can again be a problem if sealants are applied soon after overcompression.
The overcompression causes an undesired large increase in cell pressure which over time may be relieved into the sealant. Twist points in the rod can cause creases in the rod which can be extremely difficult to fill with sealant. If air is entrained and the conditions of 1 above occur, the entrained air can expand causing bubbling. Tiny voids in the concrete which if covered with sealant are entrained pockets of air which can expand with heating by the sun and cause bubbling.
Further, in the past, manufacturers of backer rods have sought to provide job-specific backer rods. Consequently, certain backer rods are specified for use with cold sealants and not for hot sealants. Others are specified for use with hot sealants only. Such specificity of product application leads to storage, order fulfillment, and quality control problems. It is readily seen that the provision of a universal backer rod as described hereinbelow resolves such problems and avoids technical considerations of selecting the specific backer rod for the application.
Expansion joint seals have not generally been viewed as unitary structures, and separate technical details of the sealants and the backer rods have been maintained. In the following, the sealant is interspersed in and within the open cells of the backer rod and becomes strongly mechanically bonded thereto. The expansion joint seal, unlike those described in the patents above, because of the mechanical bond, expand and contract as a unit and do not operate independently.
The submission of the above discussion of documents is not intended as an admission that any such document constitutes prior art against the claims of the present application. Applicant does not waive any right to take any action that would be appropriate to antedate or otherwise remove any listed document as a competent reference against the claims of the present application.